1. Technical Field
The present invention relates generally to clock and data recovery technology, and more particularly to the frequency locking technology of clock and data recovery technology.
2. Description of the Related Art
In asynchronous communication, a transmitting device transmits data in the state where a receiving device is not synchronized with the transmitting device. Accordingly, the receiving device includes a clock and data recovery circuit configured to recover a clock from a received signal waveform and then identify data.
Before transmitting actual data, the transmitting device transmits a signal specially agreed in order to enable the receiving device to detect frequency and phase, i.e., a preamble signal. While receiving the preamble signal, the receiving device must detect frequency from the preamble signal first and then find the accurate phase of a clock signal having the detected frequency. The time it takes to perform the clock recovery procedure has become a cause of the degradation of performance as communication speed has become faster.
Clock and data recovery technology may be classified into a method using reference frequency and a method of following up the frequency of a received signal without requiring reference frequency. Clock and data recovery technology may be classified into an analog method and a digital method according to the way to reduce the difference between frequency and phase. The method using reference frequency is advantageous in that frequency detection time is short, a preamble is short and its structure is simple because a previously agreed frequency can be rapidly followed up, but is disadvantageous in that the method is vulnerable to a change in the frequency of a data signal and a reference frequency generation circuit is absolutely necessary. In contrast, the method using no reference frequency is advantageous in that the method is robust against a change in the frequency of a data signal and a reference frequency generation circuit is not required, but is disadvantageous in that frequency detection time is long, a preamble must be long and its structure is complicated.